1. Field of the Invention
This invention relates to an integrated process for producing alkyl tert-butyl ethers and possibly 1-butene.
More particularly, the invention relates to an integrated process for producing methyl tert-butyl ether (MTBE) or ethyl tert-butyl ether (ETBE) and possibly 1-butene.
Still more particularly, the invention relates to maximizing the use of the butene fraction in an integrated cycle for producing MTBE or ETBE and possibly 1-butene.
2. Description of the Prior Art
Alkyl tert-butyl ether production processes are known in the art, consisting of reacting the isobutene contained in C.sub.4 hydrocarbon streams of different origin, in particular in streams originating from steam cracking, catalytic cracking or isobutane dehydrogenation plants, with an alcohol preferably chosen from methanol and ethanol. U.S. Pat. Nos. 3,979,461, 4,039,590 and 4,071,567 describe for example certain MTBE processes in which the isobutene, contained in C.sub.4 streams with may also include butadiene, is reacted with methanol in the presence of an acid ion exchange resin. Other MTBE synthesis processes are described in U.S. Pat. No. 4,475,005 and in published European patent application 470,655.
This known processes enable the isobutene to undergo virtually total conversion, leaving the other components practically unchanged to be recovered and upgraded.
British patent 2,121,407 and U.S. Pat. No. 4,513,153 for example describe an alkyl tert-butyl ether production process with an associated cycle for upgrading the residual components still present in the C.sub.4 stream leaving the etherification reactor. In these patents the C.sub.4 stream, after synthesis and separation from the tert-butyl ether, is fed to a skeleton isomerization unit for converting the 1-butene and cis and trans 2-butenes to isobutene, and recycled to the alkyl tert-butyl ether synthesis section.
Because of the fact that in such an integrated system an undesirable accumulation of inerts, represented essentially by saturated hydrocarbons such as n-butane and isobutane, takes place, a fractionation or extractive distillation section for the C.sub.4 hydrocarbon feedstock is inserted between the etherification and isomerization sections to separate the saturated from the unsaturated hydrocarbons.
However the integrated alkyl tert-butyl ether process of the known art suffers from the drawbacks of having to operate a fractionation or an extractive distillation section, which inevitably significantly increases investment and production costs.
The current process alternative is simply to bleed off. However this method also has drawbacks in that together with the inerts, unsaturated hydrocarbons consisting essentially of 1- and 2- butenes, are also bled off. These butenes in the case of the integrated cycle, represent valuable materials to be upgraded by conversion into isobutene and then in to ethers. A further drawback, of no less importance, is the need to increase the inerts concentration in the cycle to reduce the butenes lost by bleeding, this resulting in increased investment and energy consumption.
A 1-butene recovery section can also be inserted into such a scheme. Processes are known in the art for recovering 1-butene from mixtures containing it. U.S. Pat. No. 4,718,986 for example describes a process for separating 1-butene contained in a C.sub.4 stream using a distillation unit. In this patent, isobutane is separated as top product from a first column, otherwise it would contaminate the final 1-butene product recovered at high purity (&gt;99%) as top product from a second distillation column. The isobutane quantity in the feedstock influences not only investment and energy consumption relating to the first column but also the overall butene yield in that it entrains consistent 1-butene ) within the overhead bleed of the first column. ne ratio of 1/15
Again in this case, the 1-butene is separated at the expense of significant losses of valuable material in the overhead bleed of the first distillation column, which are additional to those of the bottom bleed of the second distillation column.